Recombination

Recombination marks the moment when the universe becomes transparent. As expansion continues, temperatures fall enough for electrons to bind permanently to atomic nuclei, forming the first neutral atoms.

Once electrons are no longer free, photons stop scattering constantly. Light decouples from matter and begins travelling freely through space for the first time. This released radiation persists today as the cosmic microwave background.

With light no longer trapped, the universe rapidly dims. No stars or galaxies yet exist, but matter now behaves calmly, dominated by neutral hydrogen and helium. Structure can begin to grow without interference from radiation pressure.

Recombination ends the luminous primordial phase and opens the long cosmic dark ages that follow.